Young adults diagnosed with cancer constitute a particularly vulnerable population whose medical and psychosocial needs are often overlooked. Those diagnosed with cancer between the ages of 18 and 39 face a number of increased risks including an increased risk of cardiovascular disease, second cancers, and emotional distress. Despite this, only a very small percentage of cancer research has focused on the survivorship issues of young adults. The goal of the proposed research is to address some of the unmet needs among young adult cancer survivors by developing and pilot testing a physical activity intervention for this population. Physical activity was selected as the target behavior as: 1. a high percentage of young adult cancer survivors have a sedentary lifestyle and 2. research has shown that cancer survivors who increase their physical activity experience improved cardiopulmonary function, reduced fatigue, enhanced mood and superior cancer outcomes. The intervention will be based on a previously developed, theoretically-grounded, tailored Internet intervention for sedentary adults. This previously developed intervention uses components of the Transtheoretical Model and Social Cognitive Theory to promote physical activity;for example, participants complete monthly questionnaires online and received tailored feedback reports addressing their self-efficacy, decisional balance and processes of change. The intervention website also includes features such as tips for exercising while caring for young children and local physical activity resources. Two key enhancements will be added to the intervention website in order to target the young adult cancer survivor population- 1. information pertinent to cancer survivors initiating an exercise program and 2. a peer-to-peer support component. Once these enhancements have been added, 10 young adult cancer survivors will evaluate the intervention and provide qualitative feedback. Additional revisions will be made to the intervention, as needed, based on this feedback. The intervention will then be pilot tested with a sample of 40 young adult cancer survivors. Participants will be randomly assigned to an intervention group (receiving 12 weeks of access to the intervention website) or a comparison group (receiving information on three cancer-specific Internet sites). Data on intervention feasibility and acceptability will be collected along with preliminary data on intervention effects (i.e., on physical activity level, mood, and fatigue). The latter will be used for effect size estimates when designing a future efficacy trial. Over 70,000 individuals in their twenties and thirties are diagnosed with cancer each year and there is evidence that these young adults are at increased risk (relative to older cancer survivors and the general population) for cardiovascular disease, additional malignancies, and distress. Unfortunately, the behavioral interventions for cancer survivors that might diminish some of these risks have not yet targeted young adults. The proposed research would address this oversight by developing and pilot testing an Internet-based physical activity intervention for young adult cancer survivors.